1. Field of the Invention
The present invention relates to wavelength division multiplexed optical networks configured in a ring topology and, more particularly, to amplified WDM optical ring networks having reduced amplified spontaneous emission (ASE) through selective feedback of ASE to each optical amplifier in the ring.
2. Description of the Related Art
As the need for communication signal bandwidth increases, wavelength division multiplexing (WDM) has progressively gained popularity for multiplying the transmission capacity of a single optical fiber. A review of optical networks, including WDM networks, can be found in Ramaswamni et al. Optical Networks: A Practical Perspective (Morgan Kaufman, (copyright) 1998), the disclosure of which is incorporated herein by reference. In wavelength division multiplexed optical networks, plural optical channels having different wavelengths are combined together in a single optical fiber. Typically, WDM optical networks are amplified using optical amplifiers; optical amplifiers permit the direct amplification of optical signals without the need to convert to electrical signals. In many optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs), undesirable spontaneous emission is produced; in this process, dopant atoms excited by pump lasers xe2x80x9cspontaneouslyxe2x80x9d relax from an excited level to a lower energy state. As a result of this relaxation, a photon is emitted which is then amplified by the optical amplifier, reducing the overall amplifier gain (i.e., amplifier photons which would have been available for stimulated emission involving optical channels are instead used to amplify the spontaneously-emitted photons) and producing unwanted noise.
In many optical networks, optical fibers are arranged in a series of optical rings in order to protect optical paths in the event of a fiber cut. When an optical path is disrupted, optical traffic originally configured to traverse that path is re-routed in another direction around the ring (xe2x80x9cpath diversityxe2x80x9d) in order to arrive at the intended destination. Because these optical rings may be hundreds of kilometers in circumference, plural optical amplifiers are positioned along the ring. Consequently, the problem of amplified spontaneous emission is compounded when plural amplifiers are concatenated along the optical path, each amplifier receiving the ASE of the preceding amplifiers in the amplifier chain. When the optical path is configured as a ring, this parasitic process can ultimately consume nearly all the amplifier gain as ASE from the initial amplifier is re-fed to that amplifier (and re-amplified) as it traverses the ring.
Several attempts have been made to reduce the impact of amplified spontaneous emission in optical networks. One approach is to drive the optical amplifiers in a manner such that the production of ASE is minimized. This approach is depicted in U.S. Pat. No. 5,117,303; in this patent, concatenated amplifiers are operated in the saturated state. Although this technique may be useful for systems having a single optical channel to be amplified, it is does not result in ASE reduction sufficient for WDM optical systems with large numbers of optical channels and numerous concatenated amplifiers.
Other approaches to ASE reduction in optical networks rely on optical filtering to remove the portion of the optical spectrum containing the undesired emission. For example, in U.S. Pat. No. 5,283,686 an optical amplifier is coupled with a narrow bandwidth optical filter for removing undesired spontaneous emission. In-fiber Bragg gratings coupled to an optical circulator are used to reflect optical channels to a transmission waveguide while spontaneous emission exits the optical network by passing through the Bragg gratings. In U.S. Pat. No. 5,696,615 an optical filter is optionally employed to remove the portion of the optical spectrum below 1540 nm for erbium doped fiber amplifiers at each amplifier location in order to remove ASE. Similarly, U.S. Pat. No. 5,394,265 removes ASE along with an optical service channel in between the stages of a two-stage erbium amplifier. While optical filtering is an effective way to remove ASE from an optical system, filtering only removes ASE in the wavelength region being filtered. Some forms of filtering also remove a large portion of the gain spectrum of an optical amplifier, thereby reducing the availability of optical channel wavelengths, which may be accommodated by that amplifier.
In U.S. Pat. No. 6,175,436 ASE feedback is used in a gain control technique for a series of amplifiers in a point-to-point optical network. A lasing signal is created for gain equalization. In a chain of amplifiers, only the first amplifier is equalized; the lasing signal traverses the entire length of the optical network through each of the concatenated amplifiers such that the lasing signal is amplified by each subsequent amplifier. Although this technique may be used for gain equalization in a point-to-point optical network, it cannot be used to address the problem of xe2x80x9crunawayxe2x80x9d ASE in amplified optical rings; the serial amplification of ASE through the application of the ""436 patent to an optical ring would drastically deplete available amplifier gain (as discussed below).
There is a need in the art for optical networks in which the impact of amplified spontaneous emission on network functioning is minimized while substantially preserving the optical gain spectrum of the selected optical amplifiers. Such a technique could be implemented in closed-loop amplified optical ring networks to prevent serial amplification of ASE.
The present invention improves the performance of optical networks, particularly optical ring networks, by reducing amplified spontaneous emission thereby increasing the available gain of optical amplifiers within that network. In one embodiment the invention provides a WDM optical communication system including an optical fiber ring configured to carry at least one wavelength division multiplexed optical communication signal having plural optical channels at different wavelengths. Optical amplifiers are interposed along the optical fiber ring for optically amplifying the wavelength division multiplexed optical communication signal. Each optical amplifier produces amplified spontaneous emission as well as amplifying the optical channels of the WDM signal. At each optical amplifier, amplified spontaneous emission is selected by an optical feedback selector positioned in optical communication with an amplifier output. The selected amplified spontaneous emission is fed back to each optical amplifier input via an optical feedback path. A lasing wavelength spontaneously generated by oscillation within the feedback traverses the optical ring to the next adjacent amplifier where it is dropped. A new wavelength is created at that amplifier and launched onto the optical ring where it is dropped at the next adjacent amplifier. Thus, each amplifier on the ring drops the wavelength created by each upstream amplifier, reducing amplified spontaneous emission in the optical fiber ring. Using this technique, closed-ring amplified WDM metropolitan networks can be formed without the need to xe2x80x9cbreak the ringxe2x80x9d in order to avoid uncontrolled build-up of ASE.